Mirror Modifier mirrors object incorrectly
I am not entirely sure why you would use a Mirror modifier there. The mesh is not mirrored so you can't "put them back together". When working with a Mirror modifier you only create one half of the shirt and use the Mirror modifier then to generate the other half. You created a whole t-shirt and mirrored it. Why would you even want to apply the Mirror modifier now?

If you want to get a perfectly symmetrical shirt you have to do that from the beginning by creating only a half of it.

https://www.youtube.com/watch?v=JfWfisAPEzE

I hope I could help.